Field of the Invention
This invention relates to electronic devices and to oscillator circuits and more particularly to generating clock signals for electronic devices.
Description of the Related Art
In general, electronic oscillator circuits are used to generate repetitive oscillating electronic signals for a variety of integrated circuit applications (e.g., local oscillator signals for radio frequency mixers, transmitters for generating carrier waves for radio frequency signal transmission, etc.). Referring to FIG. 1, a clock generator circuit may use a conventional tank circuit, e.g., tank circuit 102, which is a tuned circuit including inductor 104 coupled to capacitor 106. Charge flows back and forth from the capacitor plates through the inductor so the tuned circuit can store electrical energy oscillating at its resonant frequency. Amplifier circuit 108 compensates for small losses in the tank circuit to sustain oscillation. By supplying a transconductance, −Gm, that is equal and opposite to the tank losses (modeled as Gloss), amplifier 108 is able to sustain oscillation indefinitely at the resonant frequency of the tank circuit and at an amplitude determined by the amplifier.
For oscillator applications that require low power for a particular performance level, the amplifier is typically biased to reduce or eliminate any excess gain. However, that amplifier bias point may vary as a function of environmental factors (e.g., temperature, strain, aging, etc.), causing the amplitude of the output signal to vary, and therefore, substantially degrade the oscillator performance. Automatic amplitude control techniques may be used to compensate for the effects of those environmental factors. Nevertheless, target performance (e.g., low power consumption for a particular amount of phase noise) may be difficult to achieve using conventional automatic amplitude control techniques. For example, flicker noise (i.e., 1/f noise) is low frequency noise that modulates the frequency of the oscillating signal. This low frequency noise may be upconverted and may cause the oscillator to operate outside of a target operating specification. When the oscillator is included as a voltage-controlled oscillator within a low bandwidth (e.g., less than 100 kHz) phase-locked loop (PLL), the PLL may not be able to suppress the phase noise of the voltage-controlled oscillator beyond the bandwidth of the PLL. Moreover, any coupling by other sources to the voltage-controlled oscillator that occurs outside of the bandwidth of the PLL may cause deterministic frequency noise or phase noise. When the oscillator is used as a free-running reference oscillator, a low quality factor (e.g., quality factor of approximately 20) of the LC oscillator may cause substantial phase noise and substantial frequency variation due to power supply variation, bias variations caused by mechanical stress or aging of the amplifier, or effects of nearby circuitry). Accordingly, improved oscillating signal generation techniques are desired.